Almost all motor vehicles share a common basic design. A motor vehicle is comprised of a body supported by wheels, an engine to provide power, a transmission to provide multiple gear selections for accelerating the vehicle, and at least one powered axle assembly to transmit power from the transmission to the wheels of the vehicle. Throughout history, most powered axle assemblies have been a rigid one piece housing containing gears, axle shafts, and brakes with wheel attachments at the outer ends of the housing. These rigid axle assemblies are typically known as “solid” or “live” axles. The housing is typically located at the rear of the vehicles and therefore drives the rear wheels.
More and more vehicles are now using the front axle to power the wheels and these are typically known as “front wheel drive” vehicles. Vehicles which are intended to be used in off-road conditions almost always have all wheels driving i.e., both the front and rear wheels are powered thereby doubling the motive force available to power the vehicle through rough or slippery terrain. In both front wheel drive and all-wheel drive vehicles, there must be a method to transmit power through the axle assembly while at the same time being able to turn the wheels side to side as required for steering. The term drive axle assembly or steerable drive axle assembly as used above and herein is intended to mean or encompass such assemblies, and their components, that drive or power one or more front or rear wheels of a vehicle.
In all cases, the drive axle assembly has some sort of swiveling mechanism at the end of the assembly that is connected to a steering knuckle. The steering knuckle can rotate side to side to provide steering and is connected to the axle housing on one side and to a wheel support assembly on the other side. There typically is a large opening in the middle of the steering knuckle to enable a drive/axle shaft to pass through it. In addition, there is some sort of connection joint in the drive/axle shaft to allow power to be transmitted by the shaft through various angles as the wheels are steered. These connections are usually either conventional universal joints or CV (constant velocity) joints. The end of the axle/drive shaft attaches to the wheel and provides power to that particular wheel.
Until the late 20th century, the standard arrangement for a steerable drive axle assembly, particularly in all wheel drive vehicles, was a “solid” or “live” axle housing. At the ends of the housing, an inner yoke commonly called a “C” in slang terms, was attached. It is called a “C” because it looks like a “C” when looking at it from a side view. The drive/axle shaft passes through the middle of the “C” and the arms of the “C” provide the attachment points for the swiveling mechanisms that connect to the steering knuckles. Located in the middle of the “C” is the axle shaft swivel joint (Universal or CV joint).
The steering knuckle swiveling mechanisms were typically kingpin assemblies. In non-powered steering axles, the kingpin is a shaft that is held stationary by the axle housing ends and some sort of bearing is attached to the top and bottom of each steering knuckle to allow the knuckle to rotate about the kingpin. This arrangement obviously cannot be used on a powered steering axle since it has a drive/axle shaft passing through the middle of the steering knuckle. In a powered axle, there is usually a short kingpin attached to each arm of the “C” and the steering knuckle bearings attach and swivel about these short kingpins. The advantage of the kingpin style of axle assembly is that it is incredibly strong and reliable. A disadvantage is that they are relatively complicated with numerous parts required, and trained mechanics are needed to assemble or service the parts correctly.
In the late 20th century, in an effort to decrease costs along with simplifying the design and assembly, automotive manufacturers replaced the kingpins with ball joints. Ball joints consist of a spherical metal bulb with a tapered threaded stud attached to the bulb. The bulb is surrounded by some sort of bearing material and the whole assembly is encased in a metal housing. The ball joints are manufactured as an assembly at a factory and are supplied to the user as a completed unit. In use, the automotive assembler merely presses the ball joint assembly into the suspension housing and then slides the stud into a matching hole of the steering knuckle (or vice versa depending on the design). A nut is screwed onto the stud and tightened. The assembly is complete at this point. Relatively untrained assembly personnel can assemble an axle housing quickly without needing to have mechanic skills. Most vehicles, particularly passenger cars, have used this type of assembly continuously since the 1950's.
Ball joints work exceptionally well in vehicles with low suspension loads, but they typically wear and fail quickly under severe duty usage, especially in driven axles. This is because not only do the ball joints have to support the vehicle weight, road impacts, and steering forces, but additionally must resist the torque induced twisting due to power being applied to the wheels. Many off road enthusiasts equip their vehicles with oversize tires and wheels that have larger offsets with which the vehicles were originally equipped. These changes drastically increase the loads applied to the ball joints thereby shortening the lifespan. In many cases, ball joints need to be replaced in as little as 30,000 miles on a street driven vehicle or between every race on a racing vehicle. Attempts have been made to improve the design and materials of ball joints, but success has not been great, mainly due to the fact that the inherent ball joint geometry is limited in its ability to absorb random suspension loads coming from all different directions.
Another 20th century axle design feature is that the steering knuckle provided an attachment point for a wheel spindle. A spindle is a hollow tube with a flange on the end of it. The flange attaches to the knuckle. The outside of the spindle tube provides a place for wheel bearings to reside and the hole through the middle of the spindle allows the drive/axle shaft to pass through. The wheel bearings support a hub that attaches to the brakes and the wheel and they allow the wheel to spin as the vehicle drives down the road. This spindle design is extremely rugged and is completely serviceable if any parts wear out. Just like the kingpin design, the disadvantage to the design is the numerous parts that need to be assembled and adjusted after assembly.
A change that was made a few years after the change from kingpins to ball joints was the replacement of spindle mounted wheel assemblies with unit bearing assemblies. As mentioned before, spindle mounted wheel assemblies are expensive to manufacture and assemble since they consist of numerous components that have to be carefully assembled by trained mechanics. Unit bearing assemblies are simplified combined bearing and wheel hub components assembled in a factory. The units are supplied to the automotive manufacturer as a complete unit that is simply slipped into a receiver bore in the steering knuckle and typically 3 or 4 nuts are screwed on and tightened. Like the ball joints, assembly is very quick using untrained mechanics thereby speeding assembly and cutting costs.
Unit bearings have a similar problem as ball joints in that they typically don't hold up as well as previous designs under severe duty usage, especially with oversize tires and larger wheel offsets. Recognizing this problem, auto manufacturers have recently redesigned unit bearings destined for severe duty vehicles. The newer design unit bearings are manufactured with much larger and heavier components inside the unit along with spacing the bearings farther apart. All of these changes make for a much more rugged unit. The placement of the unit bearings within the steering knuckle allow for the increased size. Unlike ball joints, there is room to install essentially scaled up versions of previous unit bearings.